One access method that is utilized in communication by terminal devices in a wireless cellular system is time division multiplexing (TDM). In TDM, multiple sources are each assigned one or more different time slots in a set of time slots. Such a set of time slots is sometimes referred to as a frame. In Time Division Multiple Access (TDMA) communication, multiple terminal devices share a single frequency channel by dividing that channel into multiple time slots, with one or more time slots being assigned to each terminal device. In a typical TDMA system, the aggregate of these time slots, plus some overhead data bits, comprises a frame of data.
In a cellular or satellite system, users in other cells or beams, respectively, can cause interference among themselves when operating in the same or similar frequency bands. In the case of satellite communication, a user in one beam communicating to a satellite may be near an outer edge of this first beam and thus close to a second beam. In this case, if a user in the second beam is also at an edge of the second beam, which is close to the first beam, then these two users may produce inter-beam interference for each other. This is particularly true in the case of TDMA where inherently interference-resistant spread spectrum communication is not used. In such situations communications difficulties may arise in the forward, reverse, or both directions. In particular, with respect to users on the ground and near the edge of a beam, reverse interference depends on where the user unit and where the interfering unit are located within their respective beams.
Moreover, this inter-beam interference problem manifests itself more overtly on the reverse channel. When transmitting over the forward channel, a much smaller number of devices, i.e., gateways, will be transmitting to the satellite. In addition to typically being spaced apart from each other, these gateways transmit at a high power output. This high power output allows the signal to arrive at the satellite receiver with a relatively large signal to noise ratio. By comparison, on the reverse channel, terminal devices at the edges of separate beams may be relatively nearer to each other than are gateways. In addition, these small terminal devices typically have a relatively low power output as compared to gateways. Since these lower power output levels result in a smaller signal-to-noise ratio for terminal-originated transmissions arriving at a satellite receiver, users in separate beams may be more susceptible to inter-beam interference.